Field of the Invention
The invention relates to a cryostat arrangement for storage of a first cryogen, in particular for cooling an arrangement of superconducting magnets, having an outer jacket and a first tank with the first cryogen installed in the outer jacket as well as a second tank with a second liquid cryogen, wherein the first cryogen boils at a lower temperature than the second cryogen and wherein the first tank includes a neck tube, whose warm upper end is connected to the outer jacket at ambient temperature and whose cold lower end is connected to the first tank at a cryogenic temperature.
Such a cryostat arrangement is known from DE 10 2004 034 729 B4.
Description of the Related Art
The present invention relates in general to the field of cooling of technical systems, which should/must be kept at very low (=cryogenic) temperatures during operation. Such systems may include arrangements of superconducting magnets, such as those used in the field of magnetic resonance, for example, in MRI tomographs or NMR spectrometers. Such arrangements of superconducting magnets are usually cooled with liquid helium.
One of the development goals for superconducting magnetic systems is to reduce the consumption of liquid helium, which is equivalent to a reduction in the heat load on the helium tank in the case of bath-cooled systems.
One of the greatest contributions to the total heat load on the helium tank originates from the neck tubes, which connect the helium tank to the vacuum chamber, which is approximately at ambient temperature. Hence, thermal conduction in the neck tube is the most important source for thermal losses. Neck tubes have the function of enabling access to the helium tank in general. In the case of arrangements of superconducting magnets, this includes access for electrical connections or devices for refilling cryogens. In addition, the neck tubes serve as a vent path in the case of any excess pressure that might occur.
The neck tubes are typically made of stainless steel or some other suitable material having a low thermal conductivity. The neck tubes are usually connected at a suitable location to a heat sink, which has a higher temperature than the helium tank but provides a significantly higher cooling power at this higher temperature. A tank filled with liquid nitrogen is a typical example of such a heat sink.
To minimize the heat load on the helium tank, it is advantageous to place the connection between the neck tube and the heat sink (for example, a nitrogen tank) as high up as possible. The distance between the connection and the helium tank should therefore be maximized. However, there are practical limits to this. If the connection point is too high, the top end of the tube will be cooled down excessively and ice will form on the outside of the cryostat, which would at least be visually unattractive, or the heat load on the nitrogen tank becomes too large.
The connection between the nitrogen tank (referenced here as an example) and the neck tube is typically made of a material having a good thermal conductivity—for example, copper or aluminum.
A temperature gradient develops between the nitrogen tank and the connection point on the neck tube—due to the heat flow and the finite thermal conductivity of the connection. In practice, this typically amounts to a few degrees Kelvin, even when using materials of a high thermal conductivity (for example, aluminum or copper).
The two interfaces between the nitrogen tank and the connecting piece as well as between the connecting piece and the neck tube (“contact resistance”) make a significant contribution to the temperature gradient between the heat sink (nitrogen tank in the example selected here) and the neck tube.
The geometry of the connecting piece also contributes to develop the temperature gradient. To limit the height of the system, the upper opening of the room temperature bore in the magnet is designed to be as low as possible. To nevertheless achieve the required neck tube length, the neck tubes extend in the outer tank through so-called towers. For the best possible access to the room temperature bore, these towers are designed to be as narrow as possible, which sets tight limits for the possible design of the geometric shape of the connecting piece.
U.S. Pat. No. 3,358,472 A describes a cryostat arrangement in which a stream of liquid helium is generated. This is used first to cool a magnetic coil and subsequently—when the helium has evaporated on the coil—to cool the radiation shields. The known cryostat thus works with an evaporator, in which helium is carried around the coil and evaporated there. Nitrogen is used for shield cooling only in the storage tank (the bottom tank in the figure). There is no device here for “raising” the nitrogen.
U.S. Pat. No. 4,510,771 discloses a device, which is a cryostat with two cryogens (namely helium and nitrogen) and has an active condenser. This condenser is used for precooling a stream of helium and is driven by the compressor of the active condenser. The nitrogen serves essentially to cool a radiation shield but otherwise plays no role in the cooling of the neck tube. It is also crucial that a system free of cryogen losses is proposed here (“zero boil off”), i.e., there is no change here in the filling level in the parts wetted by liquid nitrogen. Therefore, this prior art document also does not disclose any compensation of the effects of changes in filling level.
The patent DE 10 2004 034 729 B4, which was already cited in the introduction, discloses a cryostat arrangement for storing liquid helium, with an outer jacket and a helium tank installed in it, wherein the helium tank is connected to the outer jacket on at least two suspension tubes and includes a neck tube, whose hot upper end is connected to the outer jacket and whose cold lower end is connected to the helium tank, into which a multistage cold head of the cryocondenser is installed. The helium tank is surrounded by a radiation shield, which is connected in a thermally conducting manner to the suspension tubes as well as to a contact surface on the neck tube and the helium tank. The contact surfaces on the neck tube are each connected to a radiation shield by means of a rigid or flexible permanent heat bridge in a heat-conducting manner. In one embodiment, the radiation shield is cooled with liquid nitrogen, which is present in a separate tank connected to the neck tube by the heat bridge. However, a heat exchanger, which precools the neck tube by consuming nitrogen, is not disclosed. There is also no device for “raising” the nitrogen.
The temperature gradient referenced above means that the temperature of the neck tube at the coupling point does not reach the theoretical minimum value of 77K (if nitrogen is used). This is the temperature of boiling nitrogen at a pressure of 1 bar. The temperature of the neck tube at the coupling point instead tends to be between 80K and 85K.
In first approximation, the heat load {dot over (Q)} is applied to the helium tank by heat transfer through the neck tube. At a constant neck tube cross section, this is proportional to the integral over the thermal conductivity λ of the neck tube material from the temperature of the helium tank (temperature=4.2K) to the temperature of the coupling point (temperature=TA):
      Q    .    ∼            ∫              4.2        ⁢                                  ⁢        K                    T        A              ⁢                  λ        ⁡                  (          T          )                    ·      dT      
For stainless steel, this thermal conductivity integral from 4.2K to 77K is approximately 326 W/m, and from 4.2K to 85K it is approximately 391 W/m. Thus, if it were possible to lower the temperature of the coupling point from 85K to 77K, the heat load due to thermal conduction in the neck tubes would decrease by 16% in first approximation.
Due to the declining liquid level in normal operation, the temperature of the coupling point is subject to constant changes. When the liquid level drops, the distance between the liquid surface and the coupling point increases and its temperature rises. The heat load on the helium tank thus increases when the filling level of the nitrogen tank decreases.